doc(oauth2): name the constructor parameter these classes actually take

[karpovantonme]

Four credential constructors document parameters they do not take:

/**
 * Creates an instance of AuthorizedUserCredentials.
 *
 * @param rest_client a dependency injection point. It makes it possible to
 *     mock internal REST types. This should generally not be overridden
 *     except for testing.
 */
explicit AuthorizedUserCredentials(AuthorizedUserCredentialsInfo info,
                                   Options options,
                                   HttpClientFactory client_factory);

rest_client is gone, and so is current_time_fn in the other two. The injection point they describe is now client_factory, so Doxygen drops the description as belonging to nothing and the parameter that is actually there ends up undocumented.

Same shape in AuthorizedUserCredentials, ComputeEngineCredentials, ImpersonateServiceAccountCredentials and ServiceAccountCredentials. Renamed the names, kept the wording each file already used, no behaviour touched.

[gemini-code-assist]

Code Review

This pull request updates the Doxygen documentation across several OAuth2 credentials header files to reflect the transition from using rest_client or current_time_fn parameters to client_factory. There are no review comments, and I have no feedback to provide.